This invention generally relates to systems for safely guiding the landing of aircraft. In particular, the present invention relates to means for monitoring total system error performance for Vertical Navigation (VNAV) approaches based on baro-altimetry.
Significant time and money has been invested in developing Required Navigation Performance (RNP)-based approaches. These approaches use baro-altimetry as the reference for vertical guidance. These approaches have until the current time been limited to minimum Decision Heights (DH) of 250 ft. In practice, the decision heights are often even higher than this theoretical minimum due to obstacles etc.
The current baro-altimetry-based vertical guidance systems (commonly called baro-VNAV systems) are vulnerable to certain common mode failures which could compromise safety. These include: incorrect barometric corrections settings entered by pilots, incorrect barometric corrections provided by Air Traffic Control (ATC), altitude measurement errors due to extreme temperatures and common mode failures that can affect baro-VNAV systems such as volcanic ash.
There is a perception within the industry that the baro-VNAV approach systems vertical position error cannot be bounded to a very high degree of confidence, mostly due to the common mode failure conditions discussed above.
The existing vertical guidance and/or position monitoring systems include: (1) the current baro-VNAV systems as they exist today; (2) satellite-based vertical guidance systems, in particular: (a) Space Based Augmentation Systems (SBAS, e.g., the FAA's Wide Area Augmentation System); and (b) Ground Based Augmentation Systems (GBAS, e.g., the FAA's Local Area Augmentation System (LAAS)); and (3) Enhanced Ground Proximity Warning Systems (EGPWS), which monitor only for unintentional Flight into Terrain (FIT) and not for performance relative to a defined reference path. Each of the existing options has disadvantages.
(1) Current baro-VNAV systems are available essentially 100% of the time and can theoretically work everywhere. However, these systems are always limited to higher minimums than the lowest CAT I minimums. Furthermore, the systems are vulnerable to certain common mode failures as discussed above.
(2) The disadvantages of SBAS-based vertical guidance systems are manifold. SBAS service is only available in some locations. For example, the Wide Area Augmentation System (WAAS) can provide vertical guidance sufficient to support CAT I operations but only in North America (primarily the Contiguous United States (CONUS)) and only with an availability of about 99%. Furthermore, airplane equipage to enable SBAS-based vertical guidance is costly and would be of limited or no use outside SBAS coverage.
(3) While GBAS should have high availability and should easily meet the requirements for CAT I approach operations, a GBAS ground station is still a significant investment (˜$1.5 million per site). Also, airplane equipage for GBAS (and the airplane function that uses GBAS, called GBAS Landing Systems (GLS)) is significant. The GLS function is not yet available on all models.
(4) The EGPWS does not provide guidance. It provides monitoring only. Currently EGPWS do not have knowledge of the intended or desired approach path. Consequently EGPWS do not monitor performance relative to the desired path. The monitoring by EGPWS is strictly to detect impending inadvertent flight into terrain. Currently the EGPWS monitoring is not used when the airplane gets near the ground on a precision approach.
There is a need for a system that would allow baro-altimetry-based approaches to be safely used to the lowest Category 1 minimums (i.e., 200 ft Decision Height and ½ mile Runway Visual Range (RVR)) or lower and that would allow common mode failures to be positively detected and mitigated.